Fix invalid return value for LzwInputStream

Not yet a proper part of library but its being used by people...

Author: jfreilly

src/Lzw/LzwInputStream.cs

@@ -129,7 +129,8 @@ public override int ReadByte() {
         public override int Read(byte[] buffer, int offset, int count) {
             if (!headerParsed) ParseHeader();
 
-            if (eof) return -1;
+            if (eof) return 0;
+
             int start = offset;
 
             /* Using local copies of various variables speeds things up by as